A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A mask is typically fabricated by electron beam lithography, in which a desired mask pattern is created on a photosensitive layer on the mask by means of controlled exposure of that layer to the electron beam (e-beam writing process). The pattern in the photosensitive layer is then transferred in a mask pattern layer which is typically a metal layer arranged on a transparent mask base layer.
It is known that due to the lithographic process the created mask pattern layer may exhibit some distortions or defects. Typical causes of defects are, for example, stress induced distortion (of the mask pattern layer), heating induced distortion during the e-beam writing process or distortions caused by a pellicle that is placed over the mask.
The creation of the IC involves the creation of a number of patterns overlaying each other in the target portion. The patterns overlaying each other typically have been created with different mask patterns each on a different mask. The mask patterns each have their own distortions or defects. During a transfer of the mask patterns to the target portion, the distortions will be transferred along with the mask patterns into the pattern created on the target portion. The overlaying patterns on the target portions therefore are typically associated with different distortions contributing to overlay errors within the target portion. Such overlay errors are often referred to as intra-field overlay errors.